The effects of SnO2 addition on the electrical degradation characteristics of Bi2O3-MnO2-Co3O4 -added ZnO varistors were investigated by field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and voltage-current (V-I) characteristics. The ZnO grain size was made uniform by the addition of SnO2 or Sb2O3. The nonlinearity index α of the V-I characteristic for Bi-Mn-Co-SnO2-added samples was approximately 50 and the varistor voltage was 120~140V/mm. The value of α after the electrical degradation showed a local maximum at approximately 0.1mol% added SnO2 and then showed a local minimum at approximately 0.5mol%, similar to the relative integral intensity of the XRD diffraction peak for the (004) plane for a small amount of SnO2 added. It is suggested that the diffusion of oxygen ions through the grain boundary is affected by the change in crystal orientation of ZnO grains at the grain boundary induced by the addition of a small amount of SnO2.